Sometimes one note is all you need

Many beginning songwriters and arrangers may avoid simplicity in order to avoid sounding simplistic or amateurish.  Remember that simplicity and simplistic mean two different things. Simplicity can be elegant and beautiful, while simplistic generally has a negative connotation implying a lack of depth.
Not all art needs to be complicated to be effective. In fact, there are many works that hide their lack of depth or significance in a lot of notes or lavish arrangements. It is much harder and more challenging to write an engaging work using a modicum of materials.
However, let me step back and get to the specific point of this entry, using a repeated single note in songwriting or arranging for effect. One would think that using a single note over and over would become boring, or seem amateurish and dumb.  However, in the right context it can not only work, but be emotionally effective.
A nice example of this in terms of arranging is the Three Dog Night recording of “One.” As the song builds, you’ll hear repeated octaves in the electric guitar.  Not only does this arrangement technique reflect the title of the song, the repetition of those octaves adds tension as the song builds. Repeated notes can often be used to add tension for two reasons.  First, the repetition can be thought of as a loop, and the listener can unconsciously anticipate the end of this loop, creating engagement.  Secondly, if the note is repeated over different chords, this note can form a pedal tone, meaning that the note will be harmonious with some of the harmonies, and discordant with others, building tension that can be used to build musical drama. Personally speaking, I often receive repeated notes as being insistent or assertive in nature, kind of a musical embodiment of, “I really mean this, so I’m going to say it again loudly for those of you in the back.”  When done in this manner, it is yet another way to create a form of dramatic engagement.
This previous example utilizes repeated notes in the arrangement, but not in the tune itself.  Using repeated notes in the tune is a much more dangerous proposition.  Here it can result in boredom.  However, used judiciously, repeated notes can be used effectively in songwriting.
“Every Little Thing She Does is Magic,” from the fourth Police album (Ghost in the Shell, is a wonderfully upbeat pop tune. However, the bridge of the song changes drastically, expressing how the protagonist wants to call the woman he desires, but cannot muster the courage.  The bridge ends on the markedly downbeat query, “must I always be alone?”
After the first two lines of the bridge the tune gets stuck on a single repeated note in the melody. This change comes with the first depressing line of the bridge, “but my silent fears have gripped me, long before I reach the phone.” Thus, the repeated notes reflect the protagonists mood change from ebuillient to despondent.  Also, since these repeated notes only exist at the end of the bridge, it only happens once in the song, helping to minimize any resulting boredom.  For instance if it happened in the verse or the bridge, it would be a much more dangerous situation.
“Julia” by John Lennon uses repeated notes in the verse of the song.  In fact, all but the end of the final phrase of the verse is a single note repeated.  Paired with the tempo and the delivery of the song, this colors the song with a somnolent mood that is borderline dream like.  While the chorus does have some contour, it does focus fairly heavily on two notes.  The only robust melodic contour that appears in the song is in the bridge.   However, this heavy reliance on repeated notes in the tune forces the song to lean a bit on the accompaniment, which is fortunately tenderly beautiful in its arpeggiated simplicity.
Using repeated notes in arrangements is a much safer bet than using them in songwriting.  However, keeping repeated notes as a technique in your potential bag of tricks is wise.  You can use the technique as a tool when you are trying to create a particular mood, or to create a brief section of passage that contrasts highly with the rest of your song.  Happy writing / arranging!